486 THE NERVOUS SYSTEM. 
descends on the outer side of the column of gray matter formed by the substantia 
gelatinosa Rolandi, which constitutes its terminal nucleus. Fibres constantly leave 
it to enter the nucleus, so that the lower it gets the-smaller does the spinal root 
become until, in the upper part of the spinal “cord about the level of the first or 
second spinal nerve, it disappears altogether. 
The large spinal root of the fifth nerve is a conspicuous object in sections through the 
pons and medulla. In the former it traverses the tegmental part, first, between the 
emergent part of the facial nerve and the vestibular nerve ; and then lower down between 
the restiform body and the nucleus of the facial nerve (Fig. 358, A, p. 483). In cross 
sections it presents a well-defined semilunar or curved pyriform outline. In the upper 
part of the medulla, it les on the ventral aspect of the restiform body and nearer to the 
surface (Fig. 341, p. 458). Here it is traversed and broken up into separate bundles by 
the cerebello-olivary fibres and the roots of the glosso-pharyngeal and vagus nerves. 
Finally, it comes to the surface and its fibres are spread over the area on the side of the 
megulle known as the tubercle and funiculus of Rolando (Fig. 338, p. 455). 
The small motor part of the trigeminal nerve is chiefly distributed to the muscles 
of mastication and derives its fibres from two sources, viz. from the motor nucleus 
and from the mesencephalic root of the trigeminal nerve. 
The motor nucleus (ig. 360) lies in the lateral part of the tegmental portion of 
the pons, close to the inner side of the sensory terminal nucleus, but somewhat 
nearer to the floor of the fourth ventricle. It is not placed in the exact line of the 
facial nucleus, as it is situated somewhat nearer the dorsal aspect of the pons ; but, 
nevertheless, it may be considered as being equivalent in this region to the detached 
head of the anterior horn of gray matter in the lower part of “the medulla, The 
cells of this nucleus are large and multipolar, and their axons run together to form 
the motor part of the fifth nerve. 
The mesencephalic root takes origin in the mesencephalon from a column of 
large loosely-arranged cells which are placed in the extreme lateral part of the 
eray matter which surrounds the Sylvian aqueduct. As this root is traced down- 
wards it gradually gains strength by the addition of new fibres, and it assumes a 
erescentic form in transverse section (Figs. 365, p. 494; 367, p. 496; 547, p. 467; 
346, p. 466). In the lower part of the mesencephalon it les on the inner side of 
the superior cerebellar peduncle; and the fourth nerve, on its way to the surface, 
runs downwards in its concavity and on its mesial aspect. In the upper part of 
the pons, it continues its course downwards on the outer and deep aspect of the 
gray matter which forms the floor of the fourth ventricle. Finally, reaching the 
level of the nuclei of the trigeminal nerve, the fibres of the mesencephalic root 
turn forwards and join the motor part of the trigeminal nerve. 
(1) It is not known to what parts the fibres of the mesencephalic root go. Kolliker suggests 
that they supply the tensor veli palatini and the tensor tympani ; perhaps, ‘also, they may he dis- 
tributed to the mylo-hyoid and the anterior belly of the digastric. (2) Fibres ‘from the opposite 
pyramidal tract go to the motor nucleus and bring it into connexion with the motor area of the 
cerebral cortex. (3) 3y degeneration methods the yoot of the fifth nerve has been traced down to 
the level of the second cervical nerve (Ferrier and Turner). (4) Many of the axons of the terminal 
nuclei emerge as arcuate fibres, and, proceeding through the raphe, assume a longitudinal course in 
the fillet of the opposite side, and ‘thus establish connexions with parts higher up. (5) Some of 
the axons of the cells of the terminal nucleus enter the motor nuc leus, and thus establish a simple 
reflex apparatus. 
THE DEVELOPMENT OF THE PARTS DERIVED FROM THE RHOMBENCEPHALON. 
A general sketch of the development of the medulla, pons, and cerebellum has already 
been given (p. 441). It is only necessary, therefore, in this section to call attention to 
some of the more important details connected with the process. 
Medulla.—In the embryo the cervical flexure indicates in a sharp and definite manner 
the point of junction between the cord and the brain (Fig. 327, p. 441). In the early 
condition of the rhombencephalon the calamus scriptorius extends downwards to this level, 
so that, in the first instance, there is no part of the medulla which corresponds to the 
closed portion present in the adult. The lower closed part of the medulla makes its 
appearance at a later period, and is termed by His the intercalated portion (Schaltstiick). 
In our study of the development of that part of the neural tube which forms the spinal 
